This work reports the use of a portable Raman microprobe spectrometer for the analysis of bulk and decaying compounds in carbonaceous materials such as stones, mortars and wall paintings. The analysed stones include limestone, dolomite and carbonaceous sandstone, gypsum and calcium oxalate, both mono- and dihydrated, being the main inorganic degradation products detected. Mortars include bulk phases with pure gypsum, calcite and mixtures of both or with sand, soluble salts being the most important degradation products. The pigments detected in several wall paintings include Prussian blue, iron oxide red, iron oxide yellow, vermilion, carbon black and lead white. Three different decaying processes have been characterised in the mortars of the wall paintings: (a) a massive absorption of nitrates that reacted with calcium carbonate and promoted the unbinding of pigment grains, (b) the formation of black crusts in the vault of the presbytery and (c) the thermodecomposition of pigments due to a fire.
[Show abstract][Hide abstract] ABSTRACT: Raman spectroscopy has been widely applied in the analysis of different types of artwork. This technique is sensitive, reliable, non-destructive and can be used in situ. However, there are few references in the literature regarding specific Raman spectra libraries for the field of artwork analysis. In this paper, the development of two on-line databases with Fourier transform Raman (FT-Raman; 1064 nm) and dispersive Raman (785 nm) spectra of materials used in fine art is presented; both are implemented in the e-vibrational spectroscopic databases of artists' materials database (e-VISART). The database provides not only spectra, but also information about each pigment. It must be highlighted that for each pigment or material several spectra are available from different dealers. Some of the FT-Raman spectra available in the e-VISART database have not been published until now. Some examples in which the e-VISART database has been successfully used are presented.
[Show abstract][Hide abstract] ABSTRACT: Research concerning the formation and removal of black crusts on various historical objects is approached from many different angles. The so-called "yellowing effect", observed after laser treatment for cleaning purposes, has also received a lot of attention. Evidence regarding this phenomenon differs considerably and the actual mechanisms are still speculated on by researchers. In an attempt to elucidate the processes involved in the yellowing effect associated with laser cleaning, a new analytical technique has been used to investigate the black crust, a region of the sample cleaned by laser irradiation at 1064 nm and another region of the same sample subjected to further laser irradiation at 355 nm, on a limestone sample from the cathedral of Seville in Spain. Micro-Raman spectrometry offers the advantage of spatial chemical characterization of the stone, based upon its molecular makeup and was performed on the bulk body of the stone. Raman and scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDXS) results indicate that the surfaces cleaned by irradiation at 1064 nm and by double irradiation at 1064 and 355 nm differed in terms of their calcium sulphate, calcium oxalate and iron oxide content, and that this could contribute to the difference in colour observed.
Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 10/2005; 61(11-12):2460-7. DOI:10.1016/j.saa.2004.09.010 · 2.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose a new mid-level data-fusion system to process, as a unique signal, the Raman and X-ray fluorescence (XRF) spectra obtained from the first micro-Raman–XRF instrument. The system is based on the main advantage of the wavelet transform, which is multiresolution. First, each spectrum set is split into blocks according to their frequency. The blocks which contains background and noise signals are removed and variable selection is performed on the remaining blocks to extract those variables with the most power of classification. These variables are concatenated and form a Raman–XRF meta-signal ensemble. Finally, dual-domain signal ensembles from references and samples are classified using partial least squares discriminant analysis (PLS-DA). Our results show that this system is suitable for rapidly and automatically classifying ancient pigments using the complementary information provided by both techniques. Classification with different levels of difficulty can be handled and no prior knowledge of the sample composition is required. This system has been applied to real spectra of ancient pigments and can also be applied to combinations of other spectral signals.
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